Small, Isolated Elephants Follow Own Evolutionary Path

By MARK DERR

Published: September 30, 2003

The small Borneo elephant represents the last remnant of an ancient lineage, a team of international biologists has determined. The finding, based on DNA samples, overturns a long-held prevailing theory of the animals' origins: that they were descended from domesticated elephants that reverted to the wild.

Instead, the elephant, isolated in the tropical rain forests of northeastern Borneo, has followed an independent evolutionary path for at least 18,000 years, and probably longer, the scientists conclude. In the process, it has become genetically distinct from other Asian elephants, the experts say, based on extensive comparisons of elephant DNA obtained across Asia.

The report appears in the October issue of The Public Library of Science, Biology, a new peer-reviewed, online journal that was created as a free alternative to established journals that allow access only to subscribers. (Their paper was posted in advance of its publication at biology .plosjournals.org.)

The analysis also revealed that the current Borneo elephant population, estimated at 1,000 to 2,500, lacks genetic variation, an indication that it arose from a small number of founders, was never very numerous and is highly inbred, said Dr. Prithiviraj Fernando, the main author and a scientist at the Columbia University Center for Environmental Research and Conservation.

The researchers eventually hope to learn whether the Borneo elephant -- smaller than others in Asia and more round-bodied, with larger ears and straighter tusks -- is trapped in a genetic bottleneck, caused by extensive inbreeding, or has already passed through it, as some evidence suggests, Dr. Fernando said in a telephone interview from Sri Lanka.

Extensive inbreeding can lead to undesirable genetic variants that leave the animals vulnerable to defects, diseases and changes in their environment, said Dr. Don J. Melnick, director of the Columbia center and a co-author of the study. Populations in bottlenecks often go extinct, he said.

Dr. Raman Sukumar, chairman of the World Conservation Union's Asian Elephant Specialist Group, said in an e-mail message, ''This paper is indeed very significant in that it demonstrates a certain genetic uniqueness for the elephants of Borneo, and the most likely explanation is that the elephant colonized the island during the Pleistocene and has evolved there independently.'' He was not involved in the research.

Under the prevailing theory, the Borneo elephants descended from Indian elephants presented to the Sultan of Sulu in 1750 by the East India Trading Company or were among a group shipped from Sumatra around the same time, when elephants were being heavily traded for war, logging and ceremonies from the 16th through the 18th century.

A few of those elephants were believed to have escaped or been released and reverted to the wild, while retaining the tameness of their domesticated forebears.

Asian and African elephants will attack and kill humans who threaten them, but the Borneo elephants are not aggressive toward people, said Dr. Michael Stuewe, a co-author of the paper affiliated with the Asian Rhino and Elephant Action Strategy of the World Wildlife Fund.

''I've seen researchers literally go up and touch them,'' he said. ''That tameness gave people the idea that they were zoo animals and not worth special attention.''

The elephants are found only in northeastern Borneo, in the Malaysian province of Sabah and in the Indonesian province of Kalimantan, where their forest habitat is to be protected in a park, Dr. Stuewe said. But in Sabah, logging and the conversion of forests to palm oil plantations and acacia farms for wood pulp represent a significant threat.

When the World Wildlife Fund became involved in 1999 in efforts to preserve the jungles of Sabah, ''we had to prove that we weren't wasting money on zoo animals,'' Dr. Stuewe said.

Biologists with the agency began collecting dung and blood samples from wild and captive elephants throughout Asia and shipping them to the researchers in New York.

There Dr. Fernando and his colleagues extracted and analyzed mitochondrial and autosomal DNA. Mitochondrial DNA, involved in the cell's energy system, is inherited from the mother; autosomal DNA is inherited from both parents.

Asian and African elephants and mammoths diverged from a common ancestor about six million years ago. Asian elephants ranged from the Tigris and Euphrates Rivers into China and Borneo.

Dr. Fernando had previously shown that the Asian elephant then split into two main lineages some three million years ago.

In this case, lineages reflect variations in mitochondrial DNA, dated through a genetic clock based on the rate of naturally occurring mutation.

Most mainland Asian elephants of today have diverged from the ''alpha'' lineage identified by Dr. Fernando. The elephants on the islands of Sumatra and Borneo and on the Malay Peninsula branched from the ''beta'' lineage. For reasons Dr. Fernando is now studying, Sri Lanka has elephants of both lineages.

The most recent survey shows that Borneo elephants diverged from the beta lineage 300,000 years ago. The researchers had more difficulty putting a date on their colonization of Borneo.

The lack of evidence of elephants anywhere on Borneo other than the northeast end, or of Borneo elephants anywhere else in Asia, adds to the mystery surrounding the animals' arrival and isolation.

Dr. Melnick suggested that the Borneo elephant could have diverged in its current home range and never left, the way the elephants of Sumatra and the Malay Peninsula appear to have done at a later date.

Or the lineage could have originated on the mainland, with part or all of it subsequently migrating to Borneo, as sea levels rose and fell during the Pleistocene with the advance and retreat of vast ice sheets, Dr. Fernando said. When seas were low, the Malay Peninsula and Borneo, Sumatra, Java and Bali formed a landmass called Sundaland. The scientists suggest that the elephants crossed into Borneo during one of those periods, but not since 16,000 B.C. at the peak of an ice age.

Since the glaciers melted around 10,000 B.C., the elephants have been stuck on their island. Several researchers, however, suggest that their genetic isolation began long before that.

While a number of conservation organizations have seized on the research and have begun calling the Borneo elephant a distinct subspecies, the authors reject that approach for now. ''A subspecies is difficult to define because it is subjective,'' Dr. Fernando said.

An attempt in the 1950's to classify the elephant as Elephas maximus borneensis faltered because of problems with the morphological work involving comparisons of the skeleton and other physical characteristics like tusks, Dr. Fernando said.

Until more extensive morphological and genetic studies are completed, the researchers have opted to apply the designation E.S.U., evolutionary significant unit, a reproductively isolated population that is important to the species' evolution.

By whatever name, the little Borneo elephant has suddenly been transformed from a feral animal of dubious origin to an ancient beast following its own evolutionary path in splendid isolation. The imperative for preserving it has risen accordingly.

''The Borneo elephant obviously needs special attention, and this is all the more imperative in view of the ongoing rapid attrition of its Sabah range,'' Dr. Sukumar said.

Dr. Melnick added, ''The elephants of Borneo are only going to get more different genetically if they are properly protected and managed.''

Photo: Borneo elephants are smaller than others in Asia and more round-bodied with larger ears and straighter tusks. (Photo by Marc Ancrenaz) Map of Asia highlighting former range of the Asian elephant.